How to measure speed improvements across revisions over time?

[Matthew Wilson]

I know how to use timeit and/or profile to measure the current run-time
cost of some code.

I want to record the time used by some original implementation, then
after I rewrite it, I want to find out if I made stuff faster or slower,
and by how much.

Other than me writing down numbers on a piece of paper on my desk, does
some tool that does this already exist?

If it doesn't exist, how should I build it?

Matt